Metallic implant devices, such as plates, screws, nails and pins are commonly used in the practice of orthopedic, craniofacial and cardiovascular implant surgery. Furthermore, metallic stents are also implanted into a body of a patient to support lumens, for example, coronary arteries. Most of these metallic implant devices which are currently used are constructed of stainless steel, cobalt-chromium (Co—Cr) or titanium alloys. Advantageously, these materials of construction exhibit good biomechanical properties. However, disadvantageously, implant devices constructed of these materials do not degrade over a period of time. Thus, surgery may be required when there is no longer a medical need for the implant device and when, for various reasons, it may be desired to remove the implant device from a body of a patient. For example, in certain instances, such as pediatric applications, there may be a concern that if an implant device is not removed, it may eventually be rejected by the body and cause complications for the patient. Thus, it would be advantageous for: (i) the implant device to be constructed of a material that is capable of degrading over a period of time, (ii) for the implant device to dissolve in a physiological environment such that it would not remain in the body when there is no longer a medical need for it, and (iii) surgery not to be required to remove the implant device from the body of the patient.
Currently, biomaterials used for orthopedic, craniofacial and cardiovascular applications are primarily chosen based on their ability to withstand cyclic load-bearing. Metallic biomaterials in particular have appropriate properties such as high strength, ductility, fracture toughness, hardness, corrosion resistance, formability, and biocompatibility to make them attractive for most load bearing applications. The most prevalent metals for load-bearing applications are stainless steels, Ti, and Co—Cr based alloys, though their stiffness, rigidity, and strength far exceed those of natural bone. Their elastic modulus differs significantly from bone, causing stress-shielding effects that may lead to reduced loading of bone—with this decrease in stimulation resulting in insufficient new bone growth and remodeling, decreasing implant stability. Current metallic biomaterials also suffer from the risk of releasing toxic metallic ions and particles through corrosion or wear causing implant site immune response. They may also lead to hypersensitivity, growth restriction (most significantly for pediatric implants), implant migration, and imaging interference. Due to these complications, it is estimated that 10% of patients will require a second operation for the removal of permanent metallic plates and screws, exposing patients to additional risks, and increasing surgical time and resources.
Based on at least these issues, there is a desire to design and develop a new class of load-bearing biomaterials with the goal of providing adequate support while the bone is healing that harmlessly degrades over time.
To avoid complications associated with permanent fixation implants, degradable biomaterials have recently been developed. However, resorbable polymer fixation plates and screws are relatively weaker and less rigid compared to metals, and have demonstrated local inflammatory reactions. For example, biodegradable materials which are currently used in the construction of implant devices include polymers, such as polyhydroxy acids, polylactic acid (PLA), polyglycolic acid (PGA), and the like. These materials, however, have been found to exhibit relatively poor strength and ductility, and have a tendency to react with human tissue which can limit bone growth.
Magnesium alloys have recently emerged as a new class of biodegradable materials for orthopedic applications with more comparable properties to natural bone. Magnesium is known to be a non-toxic metal element that degrades in a physiological environment and therefore, may be considered a suitable element for use in constructing biodegradable implant devices. Magnesium is attractive as a biomaterial for several reasons. It is very lightweight, with a density similar to conical bone, and much less than stainless steel, titanium alloys, and Co—Cr alloys. The elastic modulus of magnesium is much closer to natural bone compared to other commonly used metallic implants, thus reducing the risk of stress shielding. Magnesium is also essential to human metabolism, is a cofactor for many enzymes, and stabilizes the structures of DNA and RNA. Most importantly, magnesium degrades to produce a soluble, non-toxic corrosion hydroxide product which is harmlessly excreted through urine. Unfortunately, accelerated corrosion of magnesium alloys may lead to accumulation of hydrogen gas pockets around the implant as well as insufficient mechanical performance and implant stability throughout the degradation and tissue healing process. The degradation of magnesium in a physiological environment yields magnesium hydroxide and hydrogen gas. This process is known in the art as magnesium corrosion. The hydrogen gas produced in the body of the patient as a result of magnesium corrosion can produce complications because the ability of the human body to absorb or release hydrogen gas is limited.
The various biodegradable metallic alloys known in the art may exhibit low biocompatibility and/or high corrosion rates, which render these materials unsuitable for use in medical applications, such as implant devices. Further, compositions of matter for use as implant devices should not include toxic elements, such as zinc and aluminum, or at least include these elements only in non-toxic amounts. Moreover, the composition should exhibit a corrosion rate that is suitable for implantation in a physiological environment, i.e., a body of a patient.
In the field of biomedical applications, there is a desire to develop biodegradable metal alloy-containing implant materials having good compressive strength with improved corrosion resistance and biocompatibility. Further, it is desirable to control the corrosion resistance and the hydrogen evolution therefrom, which is associated with the presence of magnesium in a physiological environment.